Field of the Invention
The present invention relates to a recording medium useful in color printing, particularly for ink jet printing. More specifically, the present invention relates to a recording medium for ink jet printing comprised of a porous coating on a substrate. The present invention also relates to a method of printing using the medium of the present invention.
Ink jet printing is a printing technology in which color dots are formed on a substrate from ink droplets ejected from nozzles in a print head. The inks are generally composed of water, a water-soluble dye or a pigmented dye, one or more water-miscible cosolvents, and one or more surfactants. The substrate (ink jet medium) can be plain paper, coated paper, plastic film, cloth, and any other media which can absorb ink and form a good image. In order to form a high resolution image, the substrate is usually coated with a specially formulated ink jet coating. These coatings can be divided into two major categories, fully dense coatings and porous coatings.
The fully dense coatings are mainly comprised of film-forming polymers, with at least one of the polymers being hydrophilic. This hydrophilic polymer is either water soluble or water swellable. Sometimes a small amount of pigment is incorporated into these coatings, but the amount of pigment is usually far below the critical pigment volume concentration. This type of coating gives a glossy surface and is usually transparent. The fully dense coatings absorb ink and form an image through rapid swelling of the coating itself. The major disadvantages of this type of coating include the long ink dry time, low water resistivity of both the coating and the printed image, sensitivity of the image quality to the environment, and the difficulty in achieving a "universal" medium which would perform on all printers.
A polymeric coating is generally saturated with ink immediately after printing. This ink plasticizes the polymer coating and lowers the glass transition temperature of the coating. The coatings are tacky for a certain amount of time, usually from 30 seconds to 10 minutes, until enough solvent is evaporated from the coating to bring the glass transition temperature of the coating to near or above room temperature. During the time period, the image would smear if touched and it would block to another sheet of paper or film.
A polymeric coating needs to absorb a high amount of water rapidly to obtain a high quality image without bleeding and coalescence. On the other hand, it needs to be waterfast to provide durability. These two requirements frequently conflict with each other. It is very difficult to achieve good waterfastness and high water absorbency at the same time. The coating also needs to anchor the dye molecules in order to achieve image waterfastness. The dye molecules are water-soluble or at least water-dispersible in an aqueous ink and these same molecules need to be completely insolubilized once they are deposited and diffused into the coating. A complete insolubilization of the dye is difficult to achieve.
The polymeric ink jet coating always contains moisture and the amount of moisture depends on the environment. The imaging characteristics and ink dry time is, therefore, a function of temperature and humidity. For example, under cold and dry conditions, the equilibrium moisture in the coating is low, the free volume is also low. The initial diffusion coefficient of ink in the coating is lower than in the same media exposed to a hot and humid atmosphere. Color bleeding and coalescence can occur. In a hot and humid environment, the equilibrium moisture in the coating is high. The free volume of the coating is higher than in dry and cold conditions. Dye molecules in the ink can easily diffuse into the coating. However, the image is likely to be tacky for a long time after printing and blocking resistance of the image is expected to be low. Different environments also affect the dimensional stability of the coating due to the moisture change in the polymer. An anticurl coating is generally needed in order to balance out the dimensional change of the coating with the atmosphere. This anticurl layer adds cost to the production process.
The coating composition needs to be carefully tailored to ensure compatibility between the coating and the dye, the cosolvents in the ink, and the surfactants in the ink. For fully dense polymeric coatings, it is nearly impossible to design a "universal" ink jet medium which performs well on all or most of the commercial ink jet printers.
The second type of coating for ink jet applications is a porous coating. This type of coating is usually composed of inorganic or organic particles bonded together by a binder. The amount of pigment particles in this type of coating is often far above the critical pigment volume concentration, which results in high porosity in the coating. During the ink jet printing process, ink droplets are rapidly absorbed into the coating through capillary action and the image is dry-to-touch right after it comes out of the printer. Therefore, porous coatings allow a fast "drying" of the ink and produces a smear-resistant image. The dye molecules adsorb on the surface of the particles and form an image. High water resistance of both the coating and the image can be achieved with the porous coating. The performance of the porous ink jet coating is less sensitive to the compositions of the ink. Therefore, a universal medium which performs well on all printers can be designed. The performance of a porous coating is also much less sensitive to the temperature and humidity of the environment, so consistent imaging characteristics and dry time can be expected. The disadvantages of this type of coating, however, is the difficulty in achieving high gloss due to the high porosity in the coating.
The pigments used in ink jet coatings are usually clay (U.S. Pat. No. 4,732,786), calcium carbonate (U.S. Pat. No. 4,474,847), magnesium carbonate (U.S. Pat. Nos. 5,338,597, 5,227,962, 5,246,774), silica (UK Patent Nos. GB2129333, 2166063), surface modified silica (U.S. Pat. No. 5,372,884), zeolite, and alumina (U.S. Pat. No. 5,182,175). A combination of two or more of the above mentioned pigments can also be used. Most of these porous coatings are opaque. Therefore, dye molecules should be kept on the top surface layer in order to achieve high optical density. Pigments with high surface area are desirable, in order to keep the dye molecules on the surface layer.
Silica pigments are especially preferred in ink jet applications due to the availability of a variety of silica gels and precipitated silica with high surface area and high internal pore volume. U.S. Pat. No. 4,780,356 describes a coating composed of porous silica bonded by a water-soluble binder such as polyvinyl alcohol. The particles have a pore volume of 0.05-3.00 cc/g, a particle size of 0.1 to 5 .mu.m, and a pore size of 1 to 500 nm. U.S. Pat. No. 5,352,503 describes a coating based on silica gel with polyvinyl alcohol as the binder, polyethylene glycol as a curl-reducing agent, and a polyquaternary amine as a dye mordant.
The porous coatings composed of porous particles such as silica or zeolite possess fast drying characteristics. However, high resolution image and strong mechanical strength are difficult to obtain. The internal pores are between 1 and 50 nm for silica gel, and between 1 and 500 nm for precipitated silica. The interstitial pore size between particles is 0.4 to 3 .mu.m for 2 to 15 .mu.m particles. The nonuniform pore size distribution is a problem in that it results in differential capillary pressures within the coating, causing ink to migrate from large pores to small pores. This ink migration causes nonuniform color density, which lowers the sharpness of color tone.
The binders for these coatings are usually hydrophilic binders such as polyvinyl alcohol. The waterfastness of the coating is a function of the pigment to binder ratio. If the amount of binder is low enough so that all the polymer binder is adsorbed on the particle surface, good waterfastness can be achieved. However, the coating would have very little flexibility. This type of coating can be used with a plain paper substrate, where a thinner coating layer is required since the base paper can absorb some of the ink vehicle. In the case of impermeable substrates such as polyester and polyvinyl chloride, or low permeability substrates such as highly sized glossy paper, a thick coating (10-80 .mu.m) is required to accommodate all the ink since the coating is the sole ink absorbent. This type of coating is not suitable due to its brittleness.
When the pigment to binder ratio decreases, the toughness of the coating increases and the porosity of the coating decreases. After the particle surface is fully covered with adsorbed binder, any additional binder occupies the interstitial space. The binders adsorbed on the particle surface has limited configuration and mobility, and it is water insoluble. The other part of the binder is free polymer and it dissolves in water. As the amount of free polymer increases, the coating loses its waterfastness. The ink jet medium described in U.S. Pat. No. 5,352,503 falls into this category.
U.S. Pat. Nos. 4,879,155, 5,104,730, 5,264,275, 5,275,867 (Asahi) disclose a type of porous coating which is composed of colloidal boehmite particles bonded together by a water-soluble binder such as polyvinyl alcohol. The pore size in these coatings are controlled, so that the radius of the majority of pores lies between 1 and 10 nm. Unlike the porous coating described above, these porous coatings are transparent due to the small particle size and pore size. High optical density can be achieved whether the dye molecules are kept on the surface layer or not. Good waterfastness of both the coating layer and the printed image are achieved in this type of coating, because the polymer binder in the coating and the anionic dye in the inks are adsorbed on the surface of boehmite particles.
A porous coating composed of uniform colloidal particles can provide uniform pore size distribution, which results in high image resolution. The disadvantage of this type of coating is the low mechanical strength of the coating. The porosity only comes from interstitial pores (which are pores created by the particles themselves), i.e., the spaces between the particles, since the colloidal particles themselves are fully dense. As a result, the porosity of this type coating is usually lower than the coating composed of porous particles, which means an even thicker coating is needed to accommodate all the inks. The thicker the coating, the more difficult to overcome the brittleness of this type of coating. It is anticipated that the mechanical properties of this type of medium face severe challenges with the emergence of high ink flux and high resolution printers.
It is therefore an objective of the present invention to provide an ink jet medium which possesses high optical density and high image resolution.
It is another objective of the present invention to provide an ink jet medium which also possesses good mechanical properties, fast drying and good water fastness.
Yet another objective of the present invention is to provide an ink jet medium which provides consistent performance at different environments, as well as high optical density, high image resolution, good water fastness, fast drying and good mechanical properties.
Still another object of the present invention is to provide such an ink jet medium which is universal in that it will perform well with all printers.
These and other objects of the present invention will become apparent to the skilled artisan upon a review of the following specification. The FIGURE of the Drawing, and the claims appended hereto.